1. Field of the Invention
The present invention relates to an analog switch circuit. The circuit has a plurality of analog differential signal inputs. Therein, one of signal inputs is selected.
The invention is particularly concerned with a wideband switch for switching differential input signals of DC to several GHz. The switch has low cross-talk characteristics isolated from non-selected signal channels.
2. Description of the Prior Art
A conventional oscilloscope has a switching circuit to select an input signal channel from a plurality of analog differential input signal channels.
Prior Art 1 of a conventional analog switch circuit is shown in FIG. 1.
In FIGS. 1, 21A and 22A are the first channel differential signal input terminals, which are respectively connected to bases of transistors 1A and 2A. In like manner, 21B and 22B are the second channel differential signal input terminals, which are respectively connected to bases of transistors 1B and 2B.
Emitters of transistors 1A and 2A are connected with each other via resistors 11A and 12A. The junction point of resistors 11A and 12A is connected with a terminal of a constant current source 16A via a switch 15A. Another terminal of the source 16A is connected to a power source VEE.
Emitters of transistors 1B and 2B are connected with each other via resistors 11B and 12B. The junction point of resistors 11B and 12B is connected with a terminal of a constant current source 16B via a switch 15B. Another terminal of the source 16B is connected to the power source VEE.
Collectors of transistors 1A and 1B are connected to a power source VCC via a load resistor 33. Collectors of transistors 2A and 2B are connected to the power source VCC via a load resistor 34.
A pair of transistors 1A and 2A forms a differential amplifier. Another pair of transistors 1B and 2B forms another differential amplifier.
When the switch 15A is on and the switch 15B is off, the first pair of transistors 1A and 2A amplifies the first differential input signal between input terminals 21A and 22A to obtain a differential output between differential signal output terminals 37 and 38. The second pair of transistors 1B and 2B does not amplify the second differential input signal between input terminals 21B and 22B, because of no collector currents of transistors 1B and 2B.
When the switch 15A is off and the switch 15B is on, reversely, the second pair of transistors 1B and 2B amplifies the second differential input signal between input terminals 21B and 22B to obtain a differential output between differential signal output terminals 37 and 38. The first pair of transistors 1A and 2A does not amplify the first differential input signal between input terminals 21A and 22A, because of no collector currents of transistors 1A and 2A.
Therefore, by means of on-off switching operation of switches 15A and 15B, the analog switch circuit shown in FIG.1 can selectively amplify a signal of two differential input signals. The first signal between input terminals 21A and 22A, or the second signal between input terminals 21B and 22B, is selectable.
In FIG. 1, two differential amplifiers of two pairs of transistors 1A, 2A and 1B, 2B are shown. Many differential amplifiers with switches 15s are usable, too. When only one of switches 15s is on and the other switches 15s are off, the only one differential signal is amplified to obtain the output between terminals 37 and 38. However, the other differential input signals are not amplified.
The analog switch circuit shown in FIG. 1 has the disadvantage of cross-talk, because of base-to-collector capacitances Cbcs.
In spite of no collector current, high frequency ingredients of the input signals of bases leak out to collectors via the capacitances Cbcs.
In FIG. 2, there are shown a base-to-collector capacitance Cbc, a base-to-emitter capacitance Cbe and a collector-to-emitter capacitance Cce.
In FIG. 3, there is shown the base-to-collector capacitance Cbc depending on the collector-to-base voltage.
The base-to-emitter capacitance Cbe and the collector-to-emitter capacitance Cce, which are not shown in FIG. 3, have the same characteristics as that of the base-to-collector capacitance Cbc.
Prior Art 2 is shown in Japanese Provisional Publication No. 10-285006. Therein, an analog switch circuit is disclosed. The circuit employs means to leak high frequency ingredients for reducing the cross-talk.
In FIG. 4, the circuit of the prior art 2 is shown. The first differential signal input terminals 21A and 22A are respectively connected to bases of transistors 1A and 2A.
The second differential signal input terminals 21B and 22B are respectively connected to bases of transistors 1B and 2B.
Emitters of the transistors 1A and 2A are connected to each other via resistors 11A and 12A. The junction point of resistors 11A and 12A is connected with a terminal of a constant current source 16A via a switch 15A. Another terminal of the source 16A is connected to a power source VEE.
In like manner, emitters of the transistors 1B and 2B are connected to each other via resistors 11B and 12B. The junction point of resistors 11B and 12B is connected with a terminal of a constant current source 16B via a switch 15B. Another terminal of the source 16B is connected to.the power source VEE.
Transistors 7A, 8A, 7B and 8B are employed. In each of them, the emitter is connected with the base. The base of the transistor 7A is connected with that of 1A. In like manner, the base of 8A with 2A, 7B with 1B and 8B with 2B.
The collector of the transistor 7A is connected with that of 2A. In like manner, the collector of 8A with 1A, 7B with 2B and 8B with 1B.
Transistors 1A and 2A form a differential amplifier with means to leak high frequency ingredients for reducing the cross-talk. Transistors 7A and 8A operate as the leak means.
In like manner, transistors 1B and 2B form a differential amplifier with means to leak high frequency ingredients for reducing the cross-talk. Transistors 7B and 8B operate as the leak means.
When the switch 15A is on and the switch 15B is off, the first differential amplifier of transistors 1A and 2A amplifies the first differential input signal between input terminals 21A and 22A to obtain a differential output between differential signal output terminals 37 and 38.
The second differential amplifier of transistors 1B and 2B does not amplify the second differential input signal between the second differential input terminals 21B and 22B, because of no collector currents of transistors 1B and 2B.
However, a part of high frequency ingredients of the second differential input terminals 21B and 22B appears at collectors of transistors 1B and 2B by passing through base-to-collector capacitances Cbcs of transistors 1B and 2B.
Transistors 7B and 8B, which have base-to-collector capacitances Cbcs, leak a part of high frequency ingredients to collectors 2B and 1B in reverse phase respectively. Therefore, if the both leaks of transistors 1B and 8B are equal in their amplitude, the leaks can be canceled, because of their phase reverse to each other. In like manner, the leaks of transistors 2B and 7B can be canceled, because of their phase reverse to each other.
Actually, collector-to-base voltages of transistors 1B and 2B are not equal, and their base-to-collector capacitances are not same in value. Their capacitances vary in value depending on the collector-to-base voltages which are varied by differential input signal between input terminals 21B and 22B. Therefore, the cancellation by using leaks is not satisfied. The dispersion of base-to-collector capacitances is one of impedimental factors of the cancellation.
An object of the invention is to provide an analog switch circuit.
Another object of the invention is to provide a wideband analog switch circuit selecting a signal from differential signals without cross-talk.
A further object of the invention is to provide an analog switch circuit easy producible as a monolithic integrated circuit without cross-talk.
In the circuit of the invention, a plurality of cell switch means and a differential load means are included. Each of cell switch means includes a cell amplifying means and cell common-base means.
The cell amplifying means amplifies a differential input signal between differential input terminals to obtain a differential amplified signal between differential output terminals. The cell common-base means added with the differential amplified signal obtains a cell differential output. The output has the same polarity as or the reverse polarity to those of the differential input signal.
The differential load means supplies load currents to selected one of the plurality of cell switch means to obtain a differential signal output. The selected cell switch means is active and the others are nonactive.
Each of cell switch means includes a cell common-base means. Therefore, differential input signals added to nonactive cell switch means do not leak to differential signal output terminals.
The cell common-base means includes common-base transistors. When common-base transistors are off, input signals from emitters leak almost nothing to collectors, because grounded bases, which exist between emitters and collectors, insulate collectors from emitters.
In nonactive cell switch means, even remarkably reduced leak signals are canceled out by opposite polarity leak signals. Therefore, a wideband analog switch circuit switching signals of DC to several GHz is obtainable.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings.